1. Field of the Invention
The invention relates to a directional valve for controlling the pressure and flow of hydraulic oil to an from working connections of a consumer, wherein the pressure and flow can be controlled by a slide piston which is actuable by a drive and displaceable in a slide bore and by annular ducts connected to the piston.
2. Description of the Related Art
Such directional valves are suitable, for example, for the activation of hydraulic drives which move implements or tools on working appliances, such as harvesting machines and loaders. The hydraulic drives may in this case be, for example, single-acting plunger cylinders, double-acting synchronous or differential cylinders or oil motors for one or two directions of rotation. Directional valves for such applications are known in large numbers and in the most diverse possible versions.
A directional valve of this type is known from DE 32 25 003. The pressure medium quantity is controlled by means of clocked switching magnets in proportion to an analog input signal. In this directional valve, there are no load recording lines. The usefulness of such a directional valve is therefore greatly restricted. In one of the exemplary embodiments, three of the annular ducts of the directional valve are connected to the tank. This solution is therefore not particularly advantageous because, with a view to the dynamic behavior of the direction valve and also to the manufacturing costs, the aim must be to keep the number of annular ducts as low as possible.
A directional valve is also known from DE-A1-196 46 445 which shows a valve arrangement containing two directional valves. Each of these directional valves serves for activating a double-acting consumer. Each of the directional valves is assigned a pressure balance in each case. The common pressure balance is placed in the valve slide of each directional valve, said valve slide being designed as a hollow slide. As a result of the axial movement of the valve slide into one of the working positions A and B, this pressure balance can be assigned to one or other of the working connections A, B. Directional valves and lifting cylinders of such a type are used in mobile hydraulics, for example in agricultural appliances.
DE-A1-196 46 426 discloses an arrangement which contains two directional valves and which likewise contains the pressure balance in the valve slide designed as a hollow slide. In one of the directional valves, there is an additional control magnet, as a result of the activation of which the control piston of this directional valve is brought into a position in which the two working connections of the cylinder to be controlled are connected to one another, this being known as a floating position. It is specified, in this respect, that separate ducts for control lines and pilot valves are not necessary in order to achieve the floating position.
DE-A1-197 07 722 discloses an arrangement, by means of which the inflow and return to and from a double-acting consumer can be controlled independently of one another. This is achieved by means of a continuously controllable directional valve for the inflow and continuously controllable throttle devices from the working connection A or B to the return. Here, too, the pressure balance is arranged in the valve slide of the directional valve.
Details of a valve, to be precise its stop valves, are known from DE-A1-199 19 014. The pistons of these stop valves bear with their end faces on one another, but these possess recesses forming a pressure space. When this pressure space is acted upon by a pressure of defined magnitude, the pistons can be moved axially apart from one another. The floating position for the valve is thereby assumed.
An arrangement with a directional valve which manages without a pressure balance is known from GB-A-2 298 291. The function of the latter is achieved by alternative means, to be precise by pressure sensors in the inflow and outflow of the hydraulic consumer, a pressure sensor for the pump pressure, travel sensors in each case on a valve slide acting independently for the working connection A or B, and an electronic control, using the signals from these sensors. This also shows that a plurality of identical valve units can be assembled to form a block controlling a plurality of consumers. In the event of the failure of sensor signals, a direct control of the volume flow or of the pressure in the working connection is no longer possible. Additional means are therefore necessary in order to compensate this disadvantage to an extent such that it is at least still possible to move the working appliance into a safe position.
The above description of the known prior art shows that there is a series of solutions for achieving a comprehensive functionality of such directional valves. The aim in this context is always to provide valves which are as simply constructed as possible and can be produced cost-effectively. The functionality and, particularly, the dynamic behavior are determined by the configuration of the valve slide and of the annular ducts and connecting lines cooperating with it. This also governs the number of control edges. In this case, the behavior is also decisively influenced by dimensional tolerances. It is also always necessary to take into account that the inflow metering and the return throttling for the hydraulic consumer are to be independent of one another.